1. Field of the Invention
The invention relates to an ionization chamber which comprises a plurality of measuring field electrodes which are arranged on a substrate at a distance from one another and are provided with supply leads, and also at least one electrode which is arranged at a distance from and faces the substrate and emits charge carriers under the influence of X-rays.
2. Description of Related Art
Ionization chambers of this kind are known from EP-A 562 762 and from DE-PS 1 082 989 and are used in an X-ray system so as to switch off the X-rays after a given dose has been reached during an X-ray exposure. They are arranged between an X-ray image detector and the patient to be examined, so that it is important that the ionization chamber absorbs a minimum amount of X-rays and that the spatial absorption differences within the ionization chamber are as small as possible so as to avoid reproduction of the ionization chamber.
The space between the substrate and the electrode in the ionization chamber according to EP-A 562 762 is filled with a foam insert which has a thickness of several millimeters and is provided with windows only at the area of the measuring field electrodes, so that an air volume is present in the zone between a measuring field electrode and the facing part of the electrode. Therefore, charge carriers from the electrode can reach the measuring field electrode only at the area of the windows.
The foam insert serves to prevent the supply leads for the measuring field electrodes from being struck by charge carriers during an X-ray exposure, as otherwise the measurement would be falsified as in the ionization chamber disclosed in DE-PS 1 082 989. Moreover, the foam insert enhances the mechanical stability of the ionization chamber. The absorption of X-rays by the foam insert is greater than that of the air at the area of the measuring field electrodes, even when the foam insert has a small thickness only. In the case of soft X-rays, i.e. in the case of low voltages (for example, 40 kV) applied to the X-ray tube generating the X-rays, such a difference in absorption may cause reproduction of the ionization chamber in the X-ray image; therefore, conventional Bucky exposures are often performed without an automatic exposure control system or without an ionization chamber.
Contemporary X-ray image converters, comprising electrically readable sensors (digital image detectors), moreover, are capable of reproducing absorption differences in the X-ray image which are much smaller than those reproduced by systems used thus far which utilize an X-ray film in combination with an intensifier foil. The risk of reproduction of the ionization chamber is then particularly high.